Combined magnetron and resonant stabilizing filter

ABSTRACT

A combined magnetron and resonant stabilising filter structure formed by a multi-cavity magnetron co-axially surrounded by an outer resonant cavity, in-phase cavities being coupled to the outer cavity through coupling apertures with a relatively small coupling factor and in-phase cavities being electrically locked together by straps for operation exclusively in the pi mode.

United States Patent Pickerin A r. l, 1975 [54] COMBINED MAGNETRON ANDRESONANT 2.808.538 lO/l957 Cutler INS/39.69 X STABILIZING FILTER2.976.458 3/l96i Fcinstein.... 315/3917 3.034.014 5/1962 Drcxler315139.77 5] n nto an ug P e gq he s rd. 3.274.433 9/l966Pctcrson............ 315/3977 England 3.289.036 llll966 Downing et alINS/39.77 3.334.267 8/l967 Plumrid e ENS/39.55 [73] Asslgnee f l' Elem"valve 3.384.783 5/l968 015011. 315/3977 L1m1ted, Essex. England Filedi1973 Pn'nmry E.rmni11erSaxfield Chatmon. .lr. [3H App. N0: 424N632Attorney, Agent. or Firm-Baldwin, Wight & Brown [30] Foreign ApplicationPriority Data [57] ABSTRACT Dec. 2l. i972 Great Britain 59134/72 ACombined magnetron and resonant stabmsing filter 7 structure formed by amulti-cavity magnetron col5r] 315/3951 315/3969 axially surrounded by anouter resonant cavity. in- [Z] ..H0l 25/50 phase cavities being coupledto the outer Cavity 1 0 /337' 315/395. through coupling apertures with arelatively small cou- 3l5/39-69 pling factor and in-phase cavities beingelectrically locked together by straps for operation exclusively in [56]References Llted the mode.

UNITED STATES PATENTS 21111.1 10 9/1952 Powers 315/3977 x 7 1 Draw FgureCOMBINED MAGNETRON AND RESONANT STABILIZING FILTER This inventionrelates to magnetrons.

As is well known. the frequency spectrum of a magnetron, even when it isoperated under nearly ideal conditions. is a wide one. Thus, forexample, a typical known X-band magnetron. as employed for driving aradar. and operated with a substantially perfectly rectangular pulse l tsee. wide, will develop output power at a frequency 50 MHz differentfrom the desired centre frequency, of only about 40 dB below that at thesaid centre frequency. If, as commonly occurs in practice. the pulseshape is not good and has imperfectly sloping edges, the side bandoutput power may be con siderably larger than this in relation to theoutput power at the centre frequency.

The production of large side band output power is characteristic ofmagnetrons generally and can be a serious disadvantage in magnetrons fordriving radars, for it severely limits the number of radars which can beoperated without mutual interference, in a given area and withoutresorting to wide frequency spacing. Although the production of largeside band power can be greatly reduced or even (theoretically)eliminated by specially shaping the modulating pulse employed theadoption of a half sine wave pulse will greatly reduce side band powerwhile the use of a Gaussian pulse will (theoretically eliminate italtogether this expedient is both difficult and expensive to put intopractice. Attempts have therefore been made to overcome the problem byproviding a sharply frequency selective filter in the output channel fedfrom the magnetron. In known arrangements of this nature the magnetronis connected through a line of substantial electrical length and whichusually incorporates an impedance matching transformer. to a stabilisingcavity sharply resonant at the desired centre frequency, and outputpower to the load (the radar) is taken from this cavity. However in suchan arrangement, because the magnetron is capable of operating in any ofseveral modes, severe design diffieulties are encountered.

The present invention seeks to overcome or reduce the foregoing defectsand difficulties.

According to this invention there is provided a combined magnetron andresonant stabilising filter structure comprising a multi-cavitymagnetron co-axially surrounded by an outer resonant cavity to which itis coupled, with a relatively small coupling factor, by a plurality ofcoupling apertures in the anode wall of said magnetron. said aperturesproviding coupling between said outer cavity and in-phase cavities ofthe magnetron, and means being provided electrically to lock togetherin-phase magnetron cavities for operation exclusively in the nmode.Preferably said means are straps.

By relatively small coupling factor is meant small in relation to thecoupling factor required of the apertures used in a co-axial magnetronin which, as is well known, the co-axial cavity is used to dictate thefrequency of the inner anode 1r mode.

The coupling apertures are preferably constituted by a ring of holesthrough the magnetron anode wall which also constitutes the inner wallof the outer cavity.

Preferably there is one hole between each of the alternate cavities ofthe magnetron and the outer cavity.

In a preferred embodiment the magnetron has a cylindrical anode wall tothe inner surface of which are fixed a plurality of equally spacedradial metal vanes projecting inwards towards an axial cathode toprovide a plurality of magnetron cavities, each between two adjacentvanes; alternate vanes are connected together by a metal strap and theremaining vanes are connected together by a second metal strap; theanode wall is coaxially surrounded by an outer cylindrical wall which,with the anode wall, constitute the outer and inner walls of the outercavity; a plurality of small holes, arranged in a ring in a planeperpendicular to the axis of the whole structure and substantiallymid-way along the length thereof, are provided through the magnetronanode wall, there being one hole providing communication between theouter cavity and each of the alternate cavities of the magnetron andsituated substantially mid-way between two adjacent vanes; and an outputslot is provided in the outer wall of the outer cavity. Preferably thisslot extends parallel to the axis of the whole structure and providescommunication with the interior of a fitting adapted to receive anoutput waveguide and mounted on the outside of the outer wall of theouter cavity.

The invention is illustrated in the accompanying drawing which shows, sofar as is necessary to an understanding thereof, a preferred embodimentin broken array perspective.

Referring to the drawing, the structure therein represented provides, ina simple and robust unitary construction, a strapped magnetron and asurrounding cavity which is resonant sharply at the intended centrefrequency of operation of the magnetron. The magnetron has an axialcathode (not shown) and an anode structure consisting of a cylindricalanode wall 1 from the inner surface of which project a plurality ofradial vanes 2 which are equally spaced round the cathode. Meatl strips3 and 4 are provided, one connecting together alternate vanes and theother connecting together the remaining vanes. As will be appreciatedthe magnetron, as so far described, is a strapped multicavity magnetronof well known type with a magnetron cavity between each two adjacentvanes, alternate cavities being in-phase when operation in the 11 modeis occurring.

Co-axially surrounding the magnetron anode wall I is a cylindrical wall5. This is the outer wall of an outer resonant stabilising filter cavitysurrounding the magnetron and having the anode wall 1 as its inner wall.A plurality of small holes 6, only one of which appears in the figure,extend through the wall 1 and provide coupling, of relatively lowcoupling factor, between alternate magnetron cavities, i.e. in-phasecavities, and the outer resonant cavity. These holes lie in a ring in aplane perpendicular to the axis and substantially midway along thelength of the wall anode l and each hole is mid-way between two adjacentvanes. A slot 7, parallel with the axis, is provided in the outer wall 5and leads to a fitting 8, provided on the outside of the said wall 5,for receiving the end of an output waveguide (not shown). The wholestructure is closed by end walls (the upper one does not appear in thefigure) and is evacuated and made vacuum tight in accordance with wellknown practice.

The illustrated structure as a whole resembles, mechanically, a co-axialmagnetron but, despite this superficial resemblance, it is not one butis an ordinary magnetron surrounded by, and coupled with, a stabilisingfilter constituted by the outer cavity and operating quite differentlyfrom a co-axial magnetron. The consequence of the low coupling factorcoupling between the outer cavity and the iii-phase magnetron cavitiesis that the frequency of the ir mode of the magnetron (and also thefrequencies of other unwanted modes) is not determined by the frequencyofthe outer resonant cav ity. The straps. or other means known per sefor electri cally locking inphase magnetron cavities to secure operationexclusively in the 11 mode. are therefore essential to the invention.

The invention has important practical advantages. It provides greatreduction of undesired side band power by means of a structure which issimple. very robust. and self-contained. Although the coupling betweenthe magnetron and the outer cavity can not unfairly be regarded as aline. it is a line of almost zero length, so that the difficultiesassociated with those known arrangements in which external lineconnection to a cavityfilter is employed. are avoided. The loading ofthe magnetron anode is symmetrical. Because the coupling betweenmagnetron and outer cavity is by simple small holes. the troublesomeresonances which are experienced with co-axial magnetrons having longcoupling slots often called slot modes are almost entirely avoided or somodified that the provision of lossy material. usually necessary inco-axial magnetrons to suppress them. is not required. And. finally.acceptably low pushing and "pulling" figures. comparable with those of aco-axial magnetron. are obtainable without attendant disadvantages.notably that of slow starting.

Instead of the whole combined structure being evacuated as described theco-axial cavity could be left unevacuated or gas filled as is the casewith some co-axial magnetrons. In this case the apertures 6 would be covered by dielectric windows or the whole anode cylinder 1 surrounded by agas tight dielectric (e.g. ceramic) cylinder.

I claim:

1. A combined magnetron and resonant stabilizing filter structurecomprising a magnetron having a cathode. an anode surrounding saidcathode said anode constituted by a cylindrical wall. a plurality ofmembers extending towards the cathode from said cylindrical wall toprovide a plurality of magnetron cavities each between two adjacentmembers. and phase locking means electrically to lock together in-phasemagnetron cavities for operation exclusively in the 'rr-mode; theresonant stabilizing filter structure comprising an outer resonantcavity coaxially surrounding said nagnetron; and a plurality of couplingapertures defined by said cy lindrical wall. said apertures couplingsaid in-phase magnetron cavities to said outer cavity and beingdimensioned to provide a relatively small coupling factor whereby theoscillating frequency of the magnetron is substantially independent ofthe resonant frequency of the outer resonant cavity.

2. A magnetron and filter structure according to claim 1 in which thein-phase anode cavities are electrically locked together by straps whichform said phase locking means.

Ill

3. A magnetron and filter structure according to claim I wherein thecoupling apertures are constituted by a ring of holes through themagnetron anode outer wall which also constitutes the inner wall of theouter cavity.

4. A magnetron and filter structure according to claim 3 wherein thereis one hole between alternate cavities of the magnetron anode and theouter cavity,

5. A magnetron and filter structure according to claim I wherein saidmembers are a plurality of equally spaced radial metal vanes projectinginwards towards an axial cathode to provide the plurality of magnetroncavities, each between two adjacent vanes; said phase locking meanscomprises a first metal strap connecting alternate vanes together and asecond metal strap con necting the remaining vanes together; the anodewall is co-axially surrounded by an outer cylindrical wall which. withthe anode wall. constitute the outer and inner walls of the outercavity; said coupling apertures comprising a plurality of small holes.arranged in a ring in a plane perpendicular to the axis of the wholestructure and substantially mid-way along the length thereof. therebeing one hole providing communication between the outer cavity and eachof the alternate cavities of the magnetron and situated substantiallymid way between two adjacent vanes; and an output slot is provided inthe outer wall of the outer cavity.

6. A magnetron and filter structure according to claim 5 including afitting adapted to receive an output waveguide and mounted on theoutside of the outer wall of the outer cavity. said output slotextending parallel to the axis of the whole structure and providingcommunication with the interior of said fitting.

7. A combined magnetron and resonant stabilizing filter structurecomprising. in combination:

a magnetron having a cathode. a cylindrical wall surrounding saidcathode and defining an anode. a plurality of members extending towardssaid cathode from said cylindrical wall to provide a plurality ofmagnetron cavities each between two adjacent members. and phase lockingmeans electrically locking together those magnetron cavities which arein phase for causing said magnetron to operate exclusively in theTr-mode at an intended center frequency of operation.

said resonant stabilizing filter structure comprising wall meanscoaxially surrounding said cylindrical wall for defining therewith anouter cavity which is resonant sharply at said intended center frequencyof operation of the magnetron. and means coupling said in-phasemagnetron cavities to said outer cavity for causing the oscillatingfrequency of the magnetron to be substantially independent of theresonant frequency of said outer cavity, the means last mentionedcomprising a plurality of apertures in said cylindrical wall couplingsaid in-phase magnetron cavities to said outer cavity; and

means for taking output power from said outer cavity.

1. A combined magnetron and resonant stabilizing filter structurecomprising a magnetron having a cathode, an anode surrounding saidcathode said anode constituted by a cylindrical wall, a plurality ofmembers extending towards the cathode from said cylindrical wall toprovide a plurality of magnetron cavities each between two adjacentmembers, and phase locking means electrically to lock together in-phasemagnetron cavities for operation exclusively in the pi -mode; theresonant stabilizing filter structure comprising an outer resonantcavity coaxially surrounding said nagnetron; and a plurality of couplingapertures defined by said cylindrical wall, said apertures coupling saidin-phase magnetron cavities to said outer cavity and being dimensionedto provide a relatively small coupling factor whereby the oscillatingfrequency of the magnetron is substantially independent of the resonantfrequency of the outer resonant cavity.
 2. A magnetron and filterstructure according to claim 1 in which the in-phase anode cavities areelectrically locked together by straps which form said phase lockingmeans.
 3. A magnetron and filter structure according to claim 1 whereinthe coupling apertures are constituted by a ring of holes through themagnetron anode outer wall which also constitutes the inner wall of theouter cavity.
 4. A magnetron and filter structure according to claim 3wherein there is one hole between alternate cavities of the magnetronanode and the outer cavity.
 5. A magnetron and filter structureaccording to claim 1 wherein said members are a plurality of equallyspaced radial metal vanes projecting inwards towards an axial cathode toprovide the plurality of magnetron cavities, each between two adjacentvanes; said phase locking means comprises a first metal strap connectingalternate vanes together and a second metal strap connecting theremaining vanes together; the anode wall is co-axially surrounded by anouter cylindrical wall which, with the anodE wall, constitute the outerand inner walls of the outer cavity; said coupling apertures comprisinga plurality of small holes, arranged in a ring in a plane perpendicularto the axis of the whole structure and substantially mid-way along thelength thereof, there being one hole providing communication between theouter cavity and each of the alternate cavities of the magnetron andsituated substantially mid-way between two adjacent vanes; and an outputslot is provided in the outer wall of the outer cavity.
 6. A magnetronand filter structure according to claim 5 including a fitting adapted toreceive an output waveguide and mounted on the outside of the outer wallof the outer cavity, said output slot extending parallel to the axis ofthe whole structure and providing communication with the interior ofsaid fitting.
 7. A combined magnetron and resonant stabilizing filterstructure comprising, in combination: a magnetron having a cathode, acylindrical wall surrounding said cathode and defining an anode, aplurality of members extending towards said cathode from saidcylindrical wall to provide a plurality of magnetron cavities eachbetween two adjacent members, and phase locking means electricallylocking together those magnetron cavities which are in phase for causingsaid magnetron to operate exclusively in the pi -mode at an intendedcenter frequency of operation; said resonant stabilizing filterstructure comprising wall means coaxially surrounding said cylindricalwall for defining therewith an outer cavity which is resonant sharply atsaid intended center frequency of operation of the magnetron, and meanscoupling said in-phase magnetron cavities to said outer cavity forcausing the oscillating frequency of the magnetron to be substantiallyindependent of the resonant frequency of said outer cavity, the meanslast mentioned comprising a plurality of apertures in said cylindricalwall coupling said in-phase magnetron cavities to said outer cavity; andmeans for taking output power from said outer cavity.